It is well recognized that tires influence a number of performance characteristics of a vehicle such as traction and ride. "Ride" is a broad term used to describe the manner in which a vehicle carries its occupants. A vehicle which "rides well" is one which is relatively free of annoying factors such as continual or intermittent vibration, noise and steering disturbances.
As a tire rolls under load along a surface such as a roadway, structural nonuniformities give rise to self-excited reaction forces which vary with the rotational angle of the tire. In fairly uniform tires, the magnitudes of these force variations are sufficiently small that their effects are imperceptible to the driver and passengers. In less uniformly constructed tires, force variations can degrade the ride of a vehicle to a point of unacceptability. As vehicles, particularly automobiles have been reduced in weight to improve fuel economy, the effects of tire reaction forces have become increasingly apparent.
For these reasons, auto makers and distributors of replacement or "after market" tires insist that the tires they purchase fall within prescribed limits as to force variations. Typically, maximum force limits are specified in terms of one or more of the harmonic components of the Fourier series describing the force as an instantaneous function. Since tires cannot yet be manufactured with the degree of uniformity required to always maintain force variations within acceptable limits, it is important to have the ability to measure various types of force variation so that tires with excessive force variation can be subjected to corrective action or if need be, rejected. It is common practice to evaluate tire behavior using a tire uniformity inspection machine.
In a typical uniformity inspection machine, tires are fed by conveyor to a test station where each tire is mounted upon a chuck, inflated, and rotated while its tread surface is urged radially against the circumferential surface of a loadwheel. The loadwheel is a rigid structure which rotates freely due to its engagement with the tire along an axis of rotation parallel to that of the tire. After the distance between the centers of the tire and loadwheel are adjusted to place a desired average radial load on the tire, the distance between the center distance is held fixed for the remainder of the test. Load cells or strain gauges mounted to the loadwheel spindle in appropriate orientations then measure the reaction forces exerted by the tire on the loadwheel.
Study has shown that several types of force variations occur simultaneously as an inflated tire rolls under load. One type of force variation routinely checked by tire manufacturers is radial force variation. In a uniformity inspection machine, radial forces are directed along a line between the centers of the tire and loadwheel. In a moving vehicle, radial forces are directed vertically between the center of the tire to the road surface. Tires having excessive radial force variation cause vibration and generally impair the ride of the vehicle on which they are mounted. Another parameter of interest measured on a uniformity inspection machine is lateral force. Lateral force is measured along the axis of rotation of the tire and determines the tendency of the tire to vibrate side to side.
Over the past several years auto manufacturers have been receiving peculiar complaints concerning vehicle ride. At times, the steering wheels of affected vehicles oscillate back and forth, an effect which has been termed "nibble". When nibble occurs, the driver and sometimes the passengers feel vibration. Complaints of nibble may result in a new car being returned one or more times to the dealer for corrective action. Handling such complaints is enormously costly to auto manufacturers in terms of warranty repair costs and lost customer good will.
Nibble is puzzling in that affected cars do not exhibit the effect all the time or even at a specific speed or range of speeds. Once nibble starts, it may disappear quickly or last an entire trip. Although tire/wheel imbalance may give rise to vibrations resembling those caused by nibble, nibble occurs in cases where neither excessive imbalance nor radial nor excessive lateral force variation are present. When wheel balancing or other measures fail to eliminate a vibration problem, replacing the tires is often successful. Efforts to link nibble to sources in the vehicle other than tires have been largely without success. Therefore, auto manufacturers and tire builders have been attempting to relate nibble to a characteristic of tires which can be measured. Once this is done, standards can be established so that measurements made by different tire manufacturers on different tire uniformity test machines can be used to predict nibble and reject offending tires before complaints arise.
It has been suspected that some tires give rise to fore/aft force variations which cause nibble. Fore/aft forces are reaction forces generated by a tire rolling under load as it interacts with the road surface. The magnitude and direction of the fore/aft force varies with the rotational position of the tire so that the force tends to translate the tire alternately forward, toward the direction of travel and aft, toward the opposite direction. This theory agrees well with the observed characteristics of nibble.
Fore/Aft motion of a front tire would be transmitted back through the steering mechanism in a way tending to cause rotational oscillation of the steering wheel. If both front tires exhibit fore/aft movement, their fore/aft motion will be in phase or out of phase to some degree. When both front wheels are completely in phase so that both tires move forward and backward together, the tendencies of the tires to oscillate the steering wheel will tend to cancel so that nibble will be at a minimum. On the other hand, nibble will be most noticeable when the tires are completely out of phase so that the effects of their fore/aft motion is additive at the steering wheel. This would account for the fact that nibble is only apparent intermittently.
The perception thresholds for nibble complaints can be established empirically with tires of known characteristics. It follows that if nibble is caused by fore/aft forces, measurements of fore/aft force can be used to predict whether a given tire will result in a nibble complaint when it is mounted on a vehicle. However, since fore/aft forces are small at the low test speeds at which uniformity testing is generally conducted, it has been necessary to test tires at highway speed to measure fore/aft forces with acceptable accuracy. High speed testing is undesirable among others reasons, because of the relatively long time required to accelerate the tire to test speed. Since high speed testing must be conducted on specially designed high speed test equipment, a separate test station is required. It would be highly desirable therefore to predict fore/aft forces at low test speed so that fore/aft force prediction could be integrated with other force variation measurements in a single test sequence. Yet, previous efforts to predict fore/aft translations by attempting to measure fore/aft force at low test speeds have been largely without success.
The technique used has been to equip either the tire spindle or the loadwheel spindle of a tire uniformity machine with a strain gauge or load cell oriented in the fore/aft direction so that the load cell reports directly in force units. While such force measurements, taken on a given uniformity machine, are useful to rank a group of tires in terms of their relative predispositions to cause nibble, there is not good agreement of measured force values from one similarly instrumented uniformity machine to the next. Therefore, it has not been previously possible to establish a universal specification figure which can be used to relate fore/aft force to empirically determined perception thresholds at which nibble complaints can be expected.